Fire theory A study of Australia's native cypress trees may have extinguished the theory that bushfires lit by Aboriginal people when they arrived on the continent caused major changes to the country's vegetation.

For thousands of years, Australia's Aboriginal people have used fire to hunt and to manage the landscape. Some scientists have argued that when people first arrived in Australia they set a large number of these fires, which reshaped the country's ecosystems.

The new study, led by fire ecologist Professor David Bowman from the University of Tasmania, examines this hypothesis by analysing the genetic fingerprints of more than 1400 trees from the Callitris genus, fire-sensitive conifers found across the continent.

Analysing the genetic differences between the individual trees allowed researchers to look back through history to identify periods where genetic diversity was limited, Bowman explains.

The researchers were particularly interested to see if changes in genetic diversity correlated with three critical moments in history.

The first moment was around 45,000 years ago when humans first arrived in Australia. The second was around 18,000 years ago at the height of the Last Glacial Maximum, when large parts of the country existed under extremely dry, cold, windy conditions, particularly in outback Australia. The third milestone was about 10,000 years when the glacial period ended and warmer, wetter conditions returned.

The genetic analysis showed that in the tropical regions along the northern edge of Australia, there were no population bottlenecks, the researchers report in the Proceedings of the Royal Society B.

By contrast, in central Australia, where the ice age conditions were particularly severe, the researchers saw genetic diversity reduced as populations of the trees were driven into small "refugia" or regions where they were protected from fire and adverse conditions.

Meanwhile, in the more southerly temperate regions, tree populations were stable, and expanded their range at the end of the ice age as the climate became more amenable.

"We really saw a recapitulation of the climatic history of Australia, and the Aboriginal burning story just didn't feature," says Bowman. "We just can't see evidence for the story of ecological collapse due to the advent of Aboriginal burning."

New and powerful

Professor Chris Johnson, an ecologist from the University of Tasmania who was not involved in the study, says the new results are significant.

"It helps overturn what was, for a long time, a well accepted idea about the environmental history of Australia. It does this in a new and powerful way," he says.

By examining the history of this fire-sensitive group of plants across Australia, the paper clearly shows that "fluctuations in populations of these plants across the continent since the arrival of people were driven primarily by climate, not fire," Johnson says.

"I think we have got closer to resolving a long-running debate in Australian environmental history," he says. "Did Aboriginal use of fire cause a major restructuring of vegetation across the continent? It seems not."

For Bowman, the results suggest that the Aboriginal people may have arrived in Australia already in possession of sophisticated techniques for using fire in the landscape.

"The idea of ecological collapse resulting from people arriving and burning Australia to a crisp isn't supported by our data," he says.

"The effect of Aboriginal landscape burning is a lot more subtle. It's still important, but it's subtle and it's region-specific."